1. Field of the Invention
The present invention relates to a heat exchanger configured to perform heat exchange between heating gas, such as combustion gas, and a heat transfer tube, as well as a water heater incorporating the same.
2. Description of the Related Art
One example of a heat exchanger is described in Japanese Patent Laid-Open Publication No. 2008-298325. FIG. 14 illustrates this heat exchanger.
The heat exchanger B illustrated in FIG. 14 includes a case 9, a plurality of heat transfer tubes 8, and a guide member 7 located in a central space defined by the plurality of heat transfer tubes 8. The case 9 has a bottom wall portion 90a and a front wall portion 90b which are formed with an intake vent 91 and an exhaust vent 92 for taking in and exhausting combustion gas. The heat transfer tubes 8 are formed using spiral tubes and accommodated in the case 9. The guide member 7 has an opening 70 oriented in a rearward direction Rr. Combustion gas entering the guide member 7 through the intake vent 91 proceeds in the rearward direction Rr from the opening 70 and acts on a rear region 8A of the heat transfer tubes 8. Then, the combustion gas is reflected by a rear wall portion 90c of the case 9 and passes through gaps formed on opposite lateral sides of the guide member 7 (on the sides before and behind the guide member 7 in the figure) in a forward direction Fr. Thus, the combustion gas acts on a front region 8B of the heat transfer tubes 8. Thereafter, the combustion gas is exhausted out of the case 9 through the exhaust vent 92. This arrangement makes it possible to place the guide member 7 in the central space defined by the plurality of heat transfer tubes 8 with a high space efficiency as well as to cause the plurality of heat transfer tubes 8 to recover heat from the combustion gas.
The heat exchanger B described above, however, involves the following problems.
In the heat exchanger B, an upper portion of the guide member 7 is in contact with an upper wall portion 90d of the case 9 and, hence, no gap is provided between the guide member 7 and the upper wall portion 90d. Therefore, after the combustion gas has proceeded from the opening 70 of the guide member 7 toward the rear side within the case 7, the combustion gas does not so easily proceed toward a gap 93 between the rear region 8A of the heat transfer tubes 8 and the upper wall portion 90d. This means that the upper portion of the rear region 8A of the heat transfer tubes 8 is not sufficiently utilized in heat recovery from the combustion gas. This is not so preferable in increasing the heat recovery rate.
Further, the guide member 7 is heated to elevated temperatures by the combustion gas and the heat is transferred from the upper portion of the guide member 7 directly to the upper wall portion 90d of the case 9. Accordingly, the upper wall portion 90d is heated to considerably high temperatures. The upper wall portion 90d thus heated to such high temperatures dissipates heat in an increased amount to outside the case 9, thus resulting in an increased heat loss.